Failures that occur in rotating machines such as aircraft turbine engines can rarely be detected visually, or when they can be detected it is too late to take action. Consequently, diagnostic methods are necessary to predict failures and minimise their consequences. An attempt is thus made to anticipate part failures that could damage the machine or could leave a fragment of a broken part in the machine.
The acoustic analysis is the most frequently used among all the different physical phenomena used to demonstrate functional defects or degradation of a rotating machine. Vibrations are characteristic of the condition of the equipment, and the terms vibration signature and acoustic signature are used. Sampling and processing of the vibration signature can quickly give information about the condition of the equipment and the variation with time can be monitored without needing to disassemble the machine.
There are many engine problems that can occur in a turbine engine and can be identified from vibrations, for example rotor-stator contacts, ingress of foreign bodies, abnormal unbalanced mass, bearing defect (chipped balls or rings), gear engagement defects (worn or damaged teeth), defective auxiliary system (pump, etc.).
Before an acoustic analysis of a turbine machine can be performed, data have to be acquired over a relatively long period which makes it difficult to transfer data from the aircraft to the ground. Furthermore, it is difficult to envisage processing these data directly onboard the aircraft because such processing requires a high calculation power. This means that simplified measurements have to be made in order to detect obvious abnormal phenomena such as an out-of-balance mass or large ingested foreign bodies.
Therefore data are conventionally retrieved and analysed on the ground on a test bench, said test bench being provided with high frequency acquisition systems and powerful computers. Mechanical phenomena are easier to detect during transient phases. However, the disadvantage is that the turbine engine has to be disassembled.